Wednesday, October 09, 2013

Prolonged Coastal Storm Arriving

Radar at 4 PM This Afternoon, 10-9-2013

A prolonged coastal storm will be impacting our region for
the next several days. The energy making up this area of low pressure is that
of what was once Tropical Storm Karen. With an area of high pressure now
positioned over New England and forecast to remain nearly stationary, the area
of low pressure will only be able to move slowly northward as the high tries to
fight the system. Eventually, it should halt the northward progression of the
low which should become centered just offshore of the Delmarva. But with the
low at least expected to come as far north as the Delmarva coastline, it will
be close enough to throw rain and wind our way. The presence of a strong high
in New England and a coastal low moving up from the south will increase the
pressure gradient. This will result in a decent northeast wind and this onshore
flow will be problematic for the coastline.

The rain shield associated with the area of low pressure
should continue to lift northward throughout the night. Most of the region will
probably be within the shield by late Thursday Morning. Even if steady rain isn’t
occurring, drizzle is probably going to take over with the flow off of the
ocean. We will have to monitor how far north the rain shield lifts before it
stops its northward progression. It is still possible that the Poconos misses
out on steady rain. The computer model guidance has trended wetter over the
past 24 hours and has shifted the rain shield further north. The rain will be
heavy at times. On Friday, the rain could be on the occasional side and
eventually a transition to showers will occur. Drizzle is quite possible again
on Friday when a steady rain or heavier shower isn’t taking place.

HPC/NWS Rainfall Forecast graphic from early afternoon.

Rainfall totals will probably end up being highest south and
east of Philadelphia. 2 to 4 inches of rain is quite possible in portions of
Southern New Jersey and Delaware. North and West, amounts will taper with
northwest progression. A general 1 to 2 inches is expected here. This will be occurring
over the course of several days, so flash flooding isn’t expected. However,
some slow rises will likely occur on the smaller creeks and streams, especially
if 4 inches is realized. In Southeastern NJ and especially down towards the
Delmarva, amounts over 4 inches for a storm total isn’t out of the question. This is our best guess at the moment. Rainfall
amounts will depend on how far north the low moves before retreating later this
weekend and also on if the low decides to deepen a bit more than currently
forecast.

The persistent northeast wind element is of concern, particularly
for the coastline. This may end up being what the storm is remembered for. Repeated rounds of minor coastal flooding at the time of
high tide are likely to continue with some areas of moderate coastal flooding
possible. Already, video has shown flooding at the shore points. The prolonged northeast winds will also cause seas to build up to
heights of 6 to 12 feet offshore, with waves in the surf zone building to 6 to
8 feet. Minor to moderate beach erosion is likely going to occur from the
prolonged battering. Some tidal flooding is also possible along the Delaware
Bay. Remember, the coastline is extra vulnerable due to damage from Sandy which caused the displacement of dunes. Rebuilding continues, but many areas still don't have the protection that was once in place.

Overnight, the winds will pick up further. Gusts in the 20
to 35 MPH range across the interior are possible. At the coast, wind gusts could
easily reach 40 to 45 MPH, especially along the southeastern part of the New
Jersey Coast and along the Delaware Beaches. On Thursday and Friday, wind gusts
could exceed 30 MPH across the interior and along the coast gusts could exceed
50 MPH. The winds will begin to relax during the weekend, but they will still
be gusty. With the leaves still on the trees, some spotty power outages cannot
be ruled out. We will have to watch the
wind speeds carefully due to the aforementioned concerns about the system
moving further north than expected and the possibility that the coastal low
could deepen a bit more than the models suggest.

High pressure will slowly build south again and this will
push the coastal system back to the south over the course of the weekend. It is
looking as though it will be a slow process. As a result, drizzle and showers
may linger along with gusty winds into early next week.

Interestingly enough, if the system was a fast mover and the high wasn't to our north, this would be a nuisance event. The low pressure center on some of the modeling doesn't even drop down below 1010mb.